The invention relates to a process for producing olefins and an integrated system for producing olefins.
In recent years increasing attention is given to the exploration and utilisation of natural gas resources around the globe. A disadvantage of natural gas with respect to oil is the difficulty to transport large volumes of natural gas from the source to the market. One way of efficiently transporting natural gas is by liquefying the natural gas and to transport the liquefied natural gas (LNG). Another way is to convert the methane in the natural gas to liquid hydrocarbons using a Gas-to-Liquid process (GtL). The GtL products are typically liquid and can be transported in a similar way as traditional oil and oil products.
Besides methane, the natural gas typically comprises other hydrocarbons such as ethane, propane, and butanes. Such a natural gas is referred to as wet gas. The latter two can be added to the LPG pool, however, ethane cannot. Moreover, for various reasons the ethane content in the natural gas supplied to an LNG or GtL process is restricted and therefore a significant part of the ethane must be removed from the natural gas prior to providing the natural gas to either a LNG or GtL process.
Although, the application of ethane is limited, typically ethane is combusted in a furnace to provide heat; its corresponding olefin, ethylene, is a base chemical with a wide application and is of great commercial interest. Ethane can be converted into ethylene, e.g. using a thermal cracking process. Subsequently, the ethylene can be used to produce e.g. polyethylene, styrene, ethylene oxide or mono-ethyl-glycol. The conversion of ethane to ethylene is highly endothermic and requires significant energy input. In addition, the capex for the ethane to ethylene process, in particular the back-end work-up section, and the subsequent ethylene conversion processes is high and a minimum ethylene production capacity is required to make it economically benign. When, the ethane content in the natural gas is too low, and consequently insufficient ethane is available, the ethane/ethylene route becomes unattractive.
This problem becomes even more pronounced, in case the natural gas is withdrawn from relatively small reservoirs, especially those located in remote, isolated locations, also referred to as stranded natural gas. Of course, this stranded natural gas may be converted to LNG or GtL products. However, this requires the stranded gas reservoir to sustain a minimum production level per day in order to make the investments worthwhile. Typically, such stranded natural gas reservoirs cannot achieve sufficient production levels to sustain a GtL or LNG plant. In addition, insufficient ethane is co-produced to sustain an ethane to ethylene process and subsequent ethylene conversion processes.
It has been suggested to combine an ethane steam cracker with an Oxygenate-to-Olefin (OTO) process, which can produce additional ethylene. For instance, C. Eng et al. (C. Eng, E. Arnold, E Vora, T. Fuglerud, S. Kvisle, H. Nilsen, Integration of the UOP/HYDRO MTO Process into Ethylene plants, 10th Ethylene Producers' Conference, New Orleans, USA, 1998) have suggested to combine UOP's Methanol-to-Olefins (MTO) process with a naphtha or ethane fed steam cracker. It is mentioned that by combining both processes sufficient ethylene can be produced, while coproducing valuable propylene. A disadvantage mentioned by C. Eng et al. is the fluctuating price of methanol, which is the primary feed to the MTO reaction.
In WO 2009/039948 A2, a combined steam cracking and MTP process is suggested for preparing ethylene and propylene. According to WO 2009/039948 A2, in this process, a particular advantage is obtained by combing the back-end of both processes. The methanol feedstock is produced from methane, requiring a sufficient supply of methane.
In US2005/0038304, an integrated system for producing ethylene and propylene from an OTO system and a steam cracking system is disclosed. According to US2005/0038304, in this process, a particular advantage is obtained by combining the back-end of both processes. The methanol feedstock to the OTO process is produced from synthesis gas. However, according to US2005/0038304 the production of methanol from synthesis gas has high energy requirements due to the endothermic nature of the synthesis gas production process. Such an endothermic synthesis gas production process is normally steam methane reforming.
Methanol can be produced from hydrogen and carbon monoxide or carbon dioxide. Typically, methanol is produced from a mixture of hydrogen, carbon monoxide and carbon dioxide. In order to synthesize methanol, hydrogen, carbon monoxide and carbon dioxide should be provided in a molar ratio of at least 2, which ratio is calculated by:molar ratio=(# mol H2−# mol CO2)/(# mol CO+# mol CO2).
The feed to a methanol synthesis is typically a synthesis gas. However, such a synthesis gas of course needs to contain hydrogen, carbon monoxide and carbon dioxide in a molar ratio of at least 2. Most exothermic synthesis gas processes, however, produce a synthesis gas that is hydrogen deficient. It is not sufficient to for instance pass the hydrogen deficient synthesis gas to a water-gas-shift reactor to convert part of the carbon monoxide in the synthesis gas with water to hydrogen and carbon dioxide. As can be seen in the definition of the molar ratio herein above, such a conversion does not influence the molar ratio obtained. As described in US2005/0038304, synthesis gases, which are rich enough in hydrogen are obtained from endothermic process such as Steam Methane Reforming. In order to reduce the energy consumption required to prepare the synthesis gas, the synthesis gas is mixed with a hydrogen deficient synthesis gas, e.g. obtained from an exothermic non-catalytic partial oxidation process. The mixture is then used to synthesize methanol.
In WO2007/142739A2, a process is described for producing methanol from synthesis gas. The methanol may be used for producing olefins. In the process described in WO2007/142739A2, a hydrogen stream comprising greater than 5 mol % methane is combined with the synthesis gas. The hydrogen stream may for instance be obtained from a steam cracking process.
In US2002/0143220A1, a process is described for producing olefins. A hydrocarbon feedstock is oxidatively dehydrogenated to produced olefins and synthesis gas. The synthesis gas is converted to methanol. The methanol may be converted to ethylene.
There is a need in the art for an improved integrated ethane cracking and OTO process.
It has new been found that it is possible to produce olefins by thermally cracking ethane to an olefin and hydrogen while at the same time producing further olefins using a OTO process, wherein hydrogen obtained from the cracking process and the OTO process is used to produce at least part of the oxygenate feed to the OTO process.